Electronic circuits are often protected against wrong polarity. The task of the polarity protection is to enable a current flow only in one particular direction. A current in the opposite direction can be caused e.g. by a battery or cable connected in a wrong way in installation stage. Then the polarity protection prevents short-circuiting of the battery and destruction of components. The polarity protection secures the operation of the equipment, also during a momentary short circuit in the battery's supply line. If there is a momentary short circuit or a lowered voltage in the supply lines between the battery and the load, then the capacitors connected in parallel with the load will maintain the voltage supplied to the load, and an operating polarity protection prevents a current in the opposite direction from discharging these capacitors via the supply lines.
Polarity protection is required particularly in telecommunications equipment where even a momentary break can cause annoying disturbances in the operation. A diode is the most common and generally used component in polarity protection. Further it is well known to use as polarity protection such electronic components, which can be switched on and off. For instance transistors are such components.
In the publication DE 4 031 288 C1 a diode is located between the gate of a field effect transistor (FET) and the opposite supply line, whereby the gate capacitance of the field effect transistor (FET) can be discharged through the diode if there occurs a supply voltage with the wrong polarity. Thus said publication uses the field effect transistor (FET) as the protective component and the diode as its controlling circuit. It is also possible to use a diode directly in the polarity protection. However, a diode has a considerable loss of power, particularly at higher currents.
A positive feature of a transistor is that its operation can be controlled by one electrical signal. If there is no gate voltage, then the transistor is in its passive, non-conducting state. A sufficiently high positive gate voltage puts the transistor into the conducting state, in which particularly a channel resistance of a FET induces only a low power loss. In polarity protection the control circuit switches the transistor into the conducting state when the polarity is correct, and into the non-conducting state if the polarity is wrong.
The publication U.S. Pat. No. 5,764,465 presents a circuit where the polarity protection utilises a p-channel MOSFET. The main parts of a MOSFET can be called drain, source and gate. The presented circuit will be discussed with the aid of FIG. 1. The MOSFET 13 is connected to the transistors 12 and 15. The emitter of the transistor 12 is connected to the positive terminal 11 of the supply voltage, and its collector via the resistor 17 to the negative terminal 16 (ground) of the supply voltage. These supply voltages 11, 16 are then connected to the voltage source of a device requiring polarity protection. The base of the transistor 12 is connected to its collector. The drain (D) of the MOSFET 13 is connected to the positive terminal 11 of the power supply, and the voltage to the gate (G) is supplied via the resistor 18 from the negative terminal 16 of the power supply. The source (S) of the MOSFET 13 is connected to the output 22 of the circuit, and the output is connected to the ground 16 via the resistor 20. A capacitor 21 is connected in parallel with the resistor 20. The resistor 19 is connected to the bases of the transistors 12 and 15. The collector of the transistor 15 is connected to gate (G) of the MOSFET 13. The emitter of the transistor 15 is connected to the source (S) of the MOSFET 13. A zener diode 14 is connected between the source (S) of the MOSFET 13 and the collector of the transistor 15. In this diagram the transistors 12 and 15 form a comparison circuit. They are included in the control circuit of the MOSFET, and with the aid of them the current flow through the MOSFET 13 is prevented if the direction of the current changes. If the drain-source voltage (DS) of the MOSFET 13 changes and becomes negative, then the transistor 15 is activated. This in turn will switch off the MOSFET 13. Thus the charge is substantially maintained in the capacitor 21.
The known polarity protections realised with a MOSFET have problems regarding the complex control circuit, the expensive structure and/or the slow function. For instance telecommunication equipment often utilise large capacitors in the battery line, and a short circuit between the battery line and the power supply unit will discharge the capacitors if the system does not have an active polarity protection. The control circuit of the transistor in the polarity protection must be able to put the transistor into the non-conducting state in a sufficiently rapid manner, so that there is no time for the capacitors to discharge through the short circuit, but maintain the voltage supplied to the device.